1. Field of the Invention
The present invention relates generally to an improved straight-through rotary valve structure, and more particularly to a rotary valve structure of a trombone, which has lower frictional wear and is easy to maintain and operate.
2. Description of the Related Art
The trombone is a brass instrument. Please refer to FIG. 1, which shows the structure of a conventional trombone. The trombone mainly has a telescopic slide 40, two inner tubes 4 arranged in parallel to each other, a rotary valve main body A, a main tuning tube 5 and a tuning loop 50. The telescopic slide 40 is a crooked tube body telescopically fitted with first ends of the inner tubes 4. A mouthpiece 41 is connected to a second end of one of the inner tubes 4. A second end of the other of the inner tubes 4 is connected to an inlet of the rotary valve main body A. Two ends of the tuning loop 50 are connected to the rotary valve main body A. The main tuning tube 5 is connected between an outlet of the rotary valve main body A and a bell 51.
Referring to FIGS. 2 to 4, the rotary valve main body A is composed of a case 10, a rotary valve block 20 and an outer cover 30. The case 10 is formed with an internal conic receiving space 11 with an opening. A first extension section 12 (the outlet) is disposed on a bottom of the receiving space 11 for connecting with the main tuning tube 5. In addition, a second extension section 14 is disposed on the case 10 beside the first extension section 12 for connecting with one end of the tuning loop 50. The rotary valve block 20 is a conic body. A pivot shaft 23 is disposed at a center of a bottom section of the rotary valve block 20. The rotary valve block 20 is formed with a first flow way 21 and a second flow way 22 positioned on two sides of the pivot shaft 23. The first and second flow ways 21, 22 extend to a tip and a lateral side of the rotary valve block 20 respectively. One end of the first flow way 21 is connected to the first extension section 12, while one end of the second flow way 22 is connected to the second extension section 14. The outer cover 30 is secured to the case 10 by a retainer ring 34 to block the opening of the receiving space 11. The outer cover 30 is formed with a central shaft hole 33 passing through the outer cover 30. The pivot shaft 23 extends through the shaft hole 33 and protrudes outward from the outer cover 30. The outer cover 30 is further formed with a first perforation 31 and a second perforation 32 beside the shaft hole 33. The inner tube 4 is connected to the first perforation 31. The other end of the tuning loop 50 is connected to the second perforation 32. In practice, when the trombone is played in a common tune, a player blows airflow through the mouthpiece 41 into the inner tube 4. The airflow flows through the telescopic slide 40 back to the other inner tube 4 and then flows through the first flow way 21 of the rotary valve block 20 and the first extension section 12 of the case 10 to the main tuning tube 5. The airflow eventually comes out from the bell 51 to make sounds from the bell 51. The telescopic slide 40 can be slid to vary the length of the path of airflow so as to change the pitches. When tuned up or down, the player can shift an external switch to drive the rotary valve block 20 via the pivot shaft 23. In this case, the airflow flowing out from the inner tube 4 can flow through the second flow way 22 to the turning loop 50 and then flow through the first flow way 21 and the main tuning tube 5 to come out from the bell 51. Accordingly, the airflow will flow through the tuning loop 50 and the length of the path of airflow is lengthened to change the tune of the trombone.
In practical use, the above conventional trombone has some defects as follows:    1. The conic face of the rotary valve block 20 contacts with the inner conic face of the receiving space 11 of the case 10 with a very large frictional force therebetween. In case of poor maintenance or improper operation, it will be hard to smoothly operate the rotary valve block 20.    2. There is a very large frictional force between the rotary valve block 20 and the case 10 so that it is necessary to apply a grease between the rotary valve block 20 and the case 10 to reduce the frictional force therebetween. This complicates the maintenance process and causes inconvenience in use of the trombone.    3. When playing the trombone, the saliva of the player is easy to infiltrate between the rotary valve block 20 and the case 10. The saliva must be cleared up in time. Otherwise, the rotary valve block 20 and the case 10 are very likely to rust and damage.